Nonaqueous electrolyte, lithium secondary battery containing the same, and method for manufacturing the lithium secondary battery

Abstract

The present invention relates to a non-aqueous electrolyte composition comprising an organic solvent, a lithium salt, a cyclic carbonate additive containing a carbon-carbon double bond, and an azo initiator, wherein the azo initiator is represented by the following chemical formula 1, and the cyclic carbonate additive is contained in an amount of 0.01% by weight to 5% by weight based on the total weight of the non-aqueous electrolyte composition. JPEG2024545080000017.jpg37170 (In the above Chemical Formula 1, R1 to R4 are each independently selected from an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms, excluding the case where all of R1 to R4 are methyl groups.)

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0051005, filed on April 25, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a non-aqueous electrolyte, a lithium secondary battery containing the same, and a method for producing the lithium secondary battery. [Background technology]

[0003] In recent years, the application areas of lithium secondary batteries have rapidly expanded from power supply for electronic devices such as electrical, electronic, communication, and computer equipment to power storage and supply for large-area devices such as automobiles and power storage devices. Accordingly, there has been an increasing demand for high-capacity, high-power, and highly stable secondary batteries.

[0004] In particular, high capacity, high power output, and long life are important characteristics for lithium secondary batteries for automotive applications. To achieve high capacity, secondary batteries often use positive electrode active materials with high nickel content, which have high energy density but low stability, or are operated at high voltages.

[0005] However, when a secondary battery is operated under the above conditions, as charging and discharging proceeds, the coating formed on the surface of the positive electrode / negative electrode or the electrode surface structure deteriorates due to side reactions caused by electrolyte degradation, which can lead to the elution of transition metal ions from the surface of the positive electrode.The eluted transition metal ions are then electro-deposited on the negative electrode, reducing the passivation ability of the SEI, resulting in the degradation of the negative electrode.

[0006] Such deterioration of secondary batteries tends to be accelerated when the potential of the positive electrode increases or when the battery is exposed to high temperatures.

[0007] Therefore, in order to solve this problem, research and development efforts are being conducted to find methods that can suppress the elution of metal ions from the positive electrode, form a stable SEI film on the negative electrode, and improve the electrochemical properties at high temperatures. Summary of the Invention [Problem to be solved by the invention]

[0008] As a result of extensive research conducted to solve the above problems, an object of the present invention is to provide a nonaqueous electrolyte composition that can suppress deterioration of a positive electrode, reduce side reactions between a positive electrode and an electrolyte, and form a stable SEI film on a negative electrode.

[0009] Another object of the present invention is to provide a lithium secondary battery containing the above nonaqueous electrolyte composition and thereby exhibiting improved electrochemical properties at high temperatures, and a method for producing the same. [Means for solving the problem]

[0010] In order to achieve the above object, the present invention provides a non-aqueous electrolyte composition comprising an organic solvent, a lithium salt, a cyclic carbonate additive containing a carbon-carbon double bond, and an azo initiator, wherein the azo initiator is represented by the following chemical formula 1, and the cyclic carbonate additive is included in an amount of 0.01 wt % to 5 wt % based on the total weight of the non-aqueous electrolyte composition:

[0011] [ka]

[0012] In the above Chemical Formula 1, R1 to R4 are each independently selected from an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms, excluding the case where all of R1 to R4 are methyl groups.

[0013] According to another embodiment, the present invention provides a lithium secondary battery manufactured by injecting the nonaqueous electrolyte composition into a case that houses an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0014] According to yet another embodiment, the present invention provides a method for manufacturing a lithium secondary battery, the method comprising the steps of: preparing a lithium secondary battery into which a non-aqueous electrolyte composition containing an organic solvent, a lithium salt, a cyclic carbonate-based additive containing a carbon-carbon double bond, and an azo-based initiator is injected; and activating the lithium secondary battery to form a cyclic carbonate polymer coating on the surface of an electrode, wherein the azo-based initiator is represented by the following Chemical Formula 1, and the cyclic carbonate-based additive containing a carbon-carbon double bond is included in an amount of 0.01 wt % to 5 wt % based on the total weight of the non-aqueous electrolyte composition.

[0015] [ka]

[0016] In the above Chemical Formula 1, R1 to R4 are each independently selected from an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms, excluding the case where all of R1 to R4 are methyl groups. [Effects of the Invention]

[0017] In general, cyclic carbonate additives containing carbon-carbon double bonds are known to mainly cause a ring-opening reaction in the electrolyte of a lithium secondary battery.

[0018] In contrast, the nonaqueous electrolyte composition of the present invention includes an azo-based initiator along with a cyclic carbonate-based additive containing a carbon-carbon double bond, thereby allowing the cyclic carbonate-based additive containing a carbon-carbon double bond to predominantly undergo a polymerization reaction of the carbon-carbon double bond rather than a ring-opening reaction. The cyclic carbonate polymer coating formed by the polymerization reaction of the carbon-carbon double bond has superior durability compared to coatings formed by the ring-opening reaction of the cyclic carbonate-based additive, allowing for the formation of a resilient and strong solid electrolyte interphase (SEI) coating on the surface of the negative electrode.

[0019] Therefore, the electrolyte composition of the present invention can suppress the decrease in the passivation ability of the SEI at high temperatures and prevent the deterioration of the negative electrode. Furthermore, a lithium secondary battery containing the nonaqueous electrolyte of the present invention is stable even at high temperatures and can form an electrode-electrolyte interface with low resistance, thereby realizing a lithium secondary battery with improved performance at high temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0020] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0021] As used herein, the terms "comprises," "comprises," or "having" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the presence or possible addition of one or more other features, numbers, steps, components, or combinations thereof.

[0022] In addition, in the description of "number of carbon atoms a to b" herein, "a" and "b" refer to the number of carbon atoms contained in a specific functional group. That is, the functional group may contain "a" to "b" carbon atoms. For example, an "alkylene group having 1 to 5 carbon atoms" refers to an alkylene group containing 1 to 5 carbon atoms, i.e., -CH2-, -CH2CH2-, -CH2CH2CH2-, -CH2(CH3)CH-, -CH(CH3)CH2-, and -CH(CH3)CH2CH2-. In addition, in the present specification, the term "alkylene group" refers to a branched or unbranched divalent saturated hydrocarbon group.

[0023] Furthermore, in this specification, both the alkyl group and the alkylene group may be substituted or unsubstituted. Unless otherwise defined, the term "substituted" means that at least one hydrogen atom bonded to a carbon atom is substituted with an element other than hydrogen, such as an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 2 to 20 carbon atoms, an alkynyl group having 2 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, a cycloalkenyl group having 3 to 12 carbon atoms, a heterocycloalkyl group having 3 to 12 carbon atoms, a heterocycloalkenyl group having 3 to 12 carbon atoms, an aryloxy group having 6 to 12 carbon atoms, a halogen atom, a fluoroalkyl group having 1 to 20 carbon atoms, a nitro group, an aryl group having 6 to 20 carbon atoms, a heteroaryl group having 2 to 20 carbon atoms, or a haloaryl group having 6 to 20 carbon atoms.

[0024] The present invention will now be described in more detail.

[0025] [Nonaqueous electrolyte composition] The non-aqueous electrolyte composition according to the present invention is characterized by comprising an organic solvent, a lithium salt, a cyclic carbonate-based additive containing a carbon-carbon double bond, and an azo-based initiator.

[0026] The organic solvent contained in the nonaqueous electrolyte composition according to the present invention may include at least one organic solvent selected from the group consisting of cyclic carbonate organic solvents and linear carbonate organic solvents.

[0027] The cyclic carbonate organic solvent is a highly viscous organic solvent that has a high dielectric constant and therefore easily dissociates the lithium salt in the electrolyte. Specific examples of the cyclic carbonate organic solvent include at least one organic solvent selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, and 2,3-pentylene carbonate.

[0028] The linear carbonate organic solvent is an organic solvent having low viscosity and low dielectric constant, and may include at least one organic solvent selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0029] Specifically, the organic solvent may include at least one organic solvent selected from the group consisting of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0030] In addition, in order to produce an electrolyte having high ionic conductivity, the organic solvent may further include at least one ester organic solvent selected from the group consisting of linear ester organic solvents and cyclic ester organic solvents in addition to the at least one carbonate organic solvent selected from the group consisting of cyclic carbonate organic solvents and linear carbonate organic solvents.

[0031] Specific examples of the linear ester-based organic solvent include at least one organic solvent selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0032] The cyclic ester organic solvent may be at least one organic solvent selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0033] Meanwhile, the organic solvent may further include, as needed, any organic solvent commonly used in non-aqueous electrolytes, for example, at least one of an ether-based organic solvent, a glyme-based organic solvent, and a nitrile-based organic solvent.

[0034] The ether solvent may be any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL), and 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL), or a mixture of two or more of these, but is not limited thereto.

[0035] The glyme-based solvent has a higher dielectric constant and lower surface tension than linear carbonate-based organic solvents and is less reactive with metals, and may include at least one selected from the group consisting of dimethoxyethane (glyme, DME), diethoxyethane, diglyme, triglyme, and tetraglyme (TEGDME), but is not limited thereto.

[0036] The nitrile solvent may be one or more selected from the group consisting of acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, but is not limited thereto.

[0037] The lithium salt contained in the nonaqueous electrolyte composition according to the present invention may include one or more selected from the group consisting of LiPF, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide. The lithium salt is used as an electrolyte salt in a lithium secondary battery and serves as a medium for ion transfer.

[0038] The nonaqueous electrolyte composition according to the present invention may contain an additional lithium salt in addition to the lithium salt. The additional lithium salt usually contains, for example, Li as a cation. + and as an anion, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO2 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , AsF6 - , SbF6 - , CH3SO3 - , (CF3CF2SO2)2N - , BF2C2O4 - , BC4O8 - , PF4C2O4 -, PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CF3(CF2)7SO3 - , and SCN - At least one selected from the group consisting of:

[0039] The lithium salt may be varied as appropriate within a range that is normally usable, but to obtain the optimum effect in forming an SEI film, it may be contained in the electrolyte at a concentration of 0.1 M to 4.0 M, preferably 0.8 M to 2.5 M, and more preferably 0.8 M to 2.0 M. If the lithium salt concentration is less than 0.1 M, the amount of lithium will be insufficient, resulting in poor capacity and cycle characteristics of the lithium secondary battery. If the concentration exceeds 4.0 M, the viscosity of the nonaqueous electrolyte will increase, which may result in poor electrolyte impregnation, reduced ionic conductivity, and increased battery resistance.

[0040] The nonaqueous electrolyte composition according to the present invention includes a cyclic carbonate additive containing a carbon-carbon double bond for forming a cyclic carbonate polymer coating on the surface of an electrode in a lithium secondary battery. Specifically, the cyclic carbonate additive may be at least one selected from the group consisting of vinylene carbonate and vinylethylene carbonate.

[0041] The cyclic carbonate additive containing a carbon-carbon double bond may be contained in an amount of 0.01 to 5 wt %, preferably 0.1 to 3 wt %, and more preferably 1 to 3 wt %, based on the total amount of the nonaqueous electrolyte composition. When the cyclic carbonate additive containing a carbon-carbon double bond is contained in the above range, an SEI film of an appropriate amount is formed, which can optimize various performance characteristics of the lithium secondary battery at high temperatures.

[0042] The azo-based initiator contained in the nonaqueous electrolyte composition according to the present invention may be represented by the following Chemical Formula 1: The azo-based initiator contained in the nonaqueous electrolyte composition is a cyclic carbonate-based additive containing a carbon-carbon double bond, which allows a polymerization reaction of the carbon-carbon double bond to occur predominantly rather than a ring-opening reaction. The cyclic carbonate polymer coating formed by the polymerization reaction of the carbon-carbon double bond has superior durability compared to coatings formed by the ring-opening reaction of the cyclic carbonate-based additive, and thus can form a resilient and strong solid electrolyte interphase (SEI) coating on the surface of the negative electrode. Furthermore, the azo-based initiator of the present invention is less likely to volatilize before the formation of the cyclic carbonate polymer coating, thereby enabling a sufficient initiation reaction and suppressing various side reactions caused by the volatilized initiator.

[0043] [ka]

[0044] In the above Chemical Formula 1, R1 to R4 are each independently selected from an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms, excluding the case where all of R1 to R4 are methyl groups. Preferably, in the above Chemical Formula 1, R1 to R4 may each independently be an alkyl group having 1 to 5 carbon atoms, excluding the case where all of R1 to R4 are methyl groups.

[0045] Specifically, the azo initiator may be a compound represented by the following formula 1-1 or 1-2.

[0046] [ka]

[0047] [ka]

[0048] The azo-based initiator may be contained in an amount of 0.001 to 0.3 wt % based on the total weight of the nonaqueous electrolyte composition, preferably 0.010 to 0.25 wt %, and more preferably 0.10 to 0.25 wt %. When the azo-based initiator is contained in the above range, an SEI film of an appropriate amount is formed, which can optimize various performances of the lithium secondary battery at high temperatures.

[0049] The nonaqueous electrolyte composition of the present invention may further contain a known electrolyte additive in the nonaqueous electrolyte, as needed, to prevent the nonaqueous electrolyte from decomposing and causing the collapse of the negative electrode in a high-power environment, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and the effects of suppressing battery expansion at high temperatures.

[0050] Representative examples of such other electrolyte additives may include at least one additive for forming an SEI film selected from the group consisting of halogen-substituted carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based compounds, borate-based compounds, nitrile-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

[0051] The halogen-substituted carbonate compounds include fluoroethylene carbonate (FEC).

[0052] The sultone compound includes at least one compound selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone.

[0053] Examples of the sulfate-based compounds include ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

[0054] The phosphate-based compound may be one or more compounds selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0055] Examples of the borate-based compounds include tetraphenylborate, lithium oxalyldifluoroborate (LiODFB), and lithium bisoxalateborate (LiB(C2O4)2, LiBOB).

[0056] Examples of the nitrile compound include at least one compound selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0057] The benzene-based compound may be fluorobenzene, the amine-based compound may be triethanolamine or ethylenediamine, and the silane-based compound may be tetravinylsilane.

[0058] The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte, and examples thereof include lithium difluorophosphate (LiDFP), LiPO2F2, and LiBF4.

[0059] Among these other electrolyte additives, when a combination of 1,3-propane sultone (PS), ethylene sulfate (ESA), and lithium difluorophosphate (LiDFP) is further included, a stronger SEI film can be formed on the surface of the negative electrode during the initial activation process of the secondary battery, and the generation of gas that may be generated by decomposition of the electrolyte at high temperatures can be suppressed, thereby improving the high-temperature stability of the secondary battery.

[0060] Meanwhile, two or more of the other electrolyte additives may be mixed and used, and may be included in an amount of 0.01 wt % to 20 wt %, specifically 0.01 wt % to 10 wt %, and preferably 0.05 wt % to 5 wt %, based on the total weight of the nonaqueous electrolyte. If the content of the other electrolyte additive is less than 0.01 wt %, the effect of improving the high-temperature storage characteristics and high-temperature life characteristics of the battery is minimal. If the content of the other electrolyte additive is more than 20 wt %, excessive side reactions may occur in the electrolyte during battery charge and discharge. In particular, if the other electrolyte additive is added in an excessive amount, it may not be sufficiently decomposed at high temperatures and may remain in the electrolyte at room temperature as an unreacted substance or precipitate. This may cause side reactions that reduce the life or resistance characteristics of the secondary battery.

[0061] [Lithium secondary battery] The present invention provides a lithium secondary battery containing the nonaqueous electrolyte composition.

[0062] Specifically, the lithium secondary battery includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and the nonaqueous electrolyte composition.

[0063] The lithium secondary battery of the present invention can be manufactured by a conventional method known in the art, for example, by forming an electrode assembly in which a positive electrode, a negative electrode, and a separator are sequentially stacked between the positive electrode and the negative electrode, inserting the electrode assembly into a battery case, and injecting the nonaqueous electrolyte composition of the present invention into the battery case.

[0064] The components other than the nonaqueous electrolyte composition will be described in more detail below.

[0065] (1) Positive electrode The positive electrode may be prepared by coating a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive material, and a solvent on a positive electrode current collector.

[0066] The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

[0067] The positive electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium, specifically a lithium metal oxide containing lithium and one or more metals such as cobalt, manganese, nickel, or aluminum. More specifically, the lithium metal oxide may be a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese-based oxide (e.g., LiNi 1-Y Mn Y O2 (where 0 <Y<1)、LiMn2-Z Ni Z O4 (where 0 < Z < 2, etc.), lithium-nickel-cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1, etc.), lithium-manganese-cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2, etc.), lithium-nickel-manganese-cobalt-based oxides (e.g., Li(Ni p Co q Mn r )O2 (where 0 < p < 1, 0 < q < 1, 0 < r < 1, p + q + r = 1) or Li(Ni p1 Co q1 Mn r1 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, p1 + q1 + r1 = 2, etc.), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r2 M s2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r2, and s2 are the atomic fractions of the respective independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, and p2 + q2 + r2 + s2 = 1, etc.), etc., and any one or two or more of these compounds may be included.

[0068] Among them, from the point of being able to enhance the capacity characteristics and stability of the battery, the lithium metal oxide is LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (e.g., Li(Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni0.7 Mn 0.15 Co 0.15 )O2, and Li(Ni 0.8 Mn 0.1 Co 0.1 )O2), or lithium nickel cobalt aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.), and in consideration of the remarkable improvement effect by controlling the types and content ratios of constituent elements forming the lithium composite metal oxide, the lithium composite metal oxide may be Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, and Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc., and any one or a mixture of two or more of these may be used.

[0069] The positive electrode active material may be contained in an amount of 60 to 99% by weight, preferably 70 to 99% by weight, and more preferably 80 to 98% by weight, based on the total weight of the solid content in the positive electrode mixture slurry.

[0070] The binder is a component that assists in binding the active material and the conductive material and in binding them to the current collector.

[0071] Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene, sulfonated ethylene-propylene-diene, styrene-butadiene rubber, fluororubber, and various copolymers.

[0072] Typically, the binder may be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the solid content in the positive electrode mixture slurry.

[0073] The conductive material is a component for further improving the conductivity of the positive electrode active material.

[0074] The conductive material is a component for further improving the conductivity of the positive electrode active material and may be added in an amount of 1 wt % to 20 wt % based on the total weight of the solid content in the positive electrode slurry. Such a conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and examples thereof include carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite; conductive fibers such as carbon fiber, carbon nanotubes, and metal fiber; carbon fluoride powder; conductive powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0075] Generally, the conductive material may be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the solid content in the positive electrode mixture slurry.

[0076] The solvent may contain an organic solvent such as NMP (N-methyl-2-pyrrolidone) and may be used in an amount that provides a suitable viscosity when containing the positive electrode active material, and optionally a binder, a conductive material, etc. For example, the solvent may be contained so that the concentration of the solids including the positive electrode active material, and optionally a binder and a conductive material, is 50% by weight to 95% by weight, preferably 70% by weight to 95% by weight, and more preferably 70% by weight to 90% by weight.

[0077] (2) Negative electrode The negative electrode may be prepared by coating a negative electrode mixture slurry containing a negative electrode active material, a binder, a conductive material, and a solvent on a negative electrode current collector, or a graphite electrode made of carbon (C) or a metal itself may be used as the negative electrode.

[0078] For example, when a negative electrode is manufactured by coating the negative electrode mixture slurry onto the negative electrode current collector, the negative electrode current collector generally has a thickness of 3 μm to 500 μm. Such a negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloys, etc. may be used. Furthermore, as with the positive electrode current collector, the bonding strength of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and the negative electrode current collector may be used in various forms such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0079] The negative electrode active material may include at least one selected from the group consisting of lithium metal, a carbon material capable of reversibly intercalating / deintercalating lithium ions, a metal or an alloy of such a metal and lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0080] The carbonaceous material capable of reversibly intercalating / deintercalating lithium ions may be any carbonaceous negative electrode active material commonly used in lithium ion secondary batteries, and representative examples thereof include crystalline carbon, amorphous carbon, or a combination of these. Examples of crystalline carbon include graphite, such as amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite. Examples of amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke.

[0081] As the metal or an alloy of these metals and lithium, a metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn or an alloy of these metals and lithium can be used.

[0082] Examples of the metal composite oxide include PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li x Fe2O3 (0 ≦ x ≦ 1), Li x WO2 (0 ≦ x ≦ 1), and Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, elements of Group 1, Group 2, Group 3 of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8) can be selected from the group consisting of.

[0083] Examples of the substance capable of doping and undoping lithium include Si, SiO x (0 < x ≦ 2), Si-Y alloy (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof, and is not Si), Sn, SnO2, Sn-Y (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof, and is not Sn), etc. Also, at least one of these and SiO2 may be mixed and used. The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and a combination thereof.

[0084] Examples of the transition metal oxide include lithium-containing titanium composite oxide (LTO), vanadium oxide, lithium vanadium oxide, and the like.

[0085] The additive according to the present invention is particularly effective when Si or SiO x (0 < x ≤ 2) is used as the negative electrode active material. Specifically, when a Si-based negative electrode active material is used, if a strong SEI layer is not formed on the surface of the negative electrode during initial activation, the decrease in life characteristics is promoted by intense volume expansion - contraction during the progress of the cycle. However, since the additive according to the present invention has elasticity and can form a strong SEI layer, a secondary battery using a Si-based negative electrode active material can have excellent life characteristics and storage characteristics.

[0086] The negative electrode active material may be contained at 50% to 99% by weight, preferably 60% to 99% by weight, more preferably 70% to 98% by weight, based on the total weight of the solid content in the negative electrode binder slurry.

[0087] The binder is a component that assists in binding between the conductive material, the active material, and the current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene, sulfonated ethylene - propylene - diene, styrene - butadiene rubber, fluorine rubber, various copolymers thereof, and the like.

[0088] Generally, the binder may be contained at 1% to 20% by weight, preferably 1% to 15% by weight, more preferably 1% to 10% by weight, based on the total weight of the solid content in the negative electrode binder slurry.

[0089] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 1 wt % to 20 wt % based on the total weight of the solid content in the negative electrode slurry. Such a conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and examples thereof include carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite; conductive fibers such as carbon nanotubes, carbon fibers, and metal fibers; carbon fluoride powders; conductive powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0090] The conductive material may be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the solid content in the negative electrode mixture slurry.

[0091] The solvent may include water or an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a suitable viscosity when containing the negative electrode active material, and optionally a binder and a conductive material, etc. For example, the solvent may be included so that the concentration of the solids including the negative electrode active material, and optionally a binder and a conductive material, is 50% by weight to 95% by weight, preferably 70% by weight to 90% by weight.

[0092] When a metal is used as the anode, the anode can be fabricated by physically bonding, rolling, or depositing a metal on a metal thin film or the anode current collector. The deposition method can be electrochemical deposition or chemical vapor deposition.

[0093] For example, the metal thin film itself or the metal bonded / rolled / deposited on the negative electrode current collector may include one metal or an alloy of two metals selected from the group consisting of lithium (Li), nickel (Ni), tin (Sn), copper (Cu), and indium (In).

[0094] (3) Separator The separator may be a conventional porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, either alone or in a laminate, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of a high-melting point glass fiber or polyethylene terephthalate fiber, but is not limited to these. Furthermore, to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and may be selectively used as a single-layer or multi-layer structure.

[0095] The external shape of the lithium secondary battery of the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, or the like.

[0096] [Secondary battery manufacturing method] A method for manufacturing a lithium secondary battery according to the present invention may include the steps of preparing a lithium secondary battery into which a nonaqueous electrolyte composition containing an organic solvent, a lithium salt, a cyclic carbonate-based additive containing a carbon-carbon double bond, and an azo-based initiator is injected, and activating the lithium secondary battery to form a cyclic carbonate polymer coating on the surface of an electrode.

[0097] The lithium secondary battery may be prepared by a conventional method known in the art. For example, the lithium secondary battery may be prepared by forming an electrode assembly in which a positive electrode, a negative electrode, and a separator are sequentially stacked between the positive and negative electrodes, inserting the electrode assembly into a battery case, and injecting the nonaqueous electrolyte composition of the present invention. The nonaqueous electrolyte composition of the present invention may be prepared by mixing the above-described organic solvent, lithium salt, a cyclic carbonate-based additive containing a carbon-carbon double bond, and an azo-based initiator. Since the components of the nonaqueous electrolyte composition and the electrode assembly are as described above, a description thereof will be omitted, and the other steps will be described below.

[0098] The nonaqueous electrolyte composition of the present invention can form a stronger anode protective coating on the surface of the anode by forming a cyclic carbonate polymer coating through a polymerization reaction between carbon-carbon double bonds contained in the cyclic carbonate additive during the initial activation process of the lithium secondary battery. The polymerization reaction of the carbon-carbon double bonds is promoted by the azo initiator and occurs preferentially over the ring-opening reaction of the cyclic carbonate additive.

[0099] The step of activating the lithium secondary battery may include an initial charging process, an aging process, and a discharging process.

[0100] The initial charging may be performed under conditions such that the state of charge (SOC) is 5% to 85%, preferably 5% to 60%, and most preferably 30% to 50%.

[0101] The initial charge may be carried out at a temperature of 30°C to 80°C, preferably 40°C to 70°C, and more preferably 45°C to 70°C.

[0102] The aging process may be carried out at a high temperature of 30°C to 80°C, preferably 40°C to 70°C, and more preferably 45°C to 70°C. High-temperature aging has the advantage of stabilizing the electrode coating and facilitating the release of gas generated within the electrode. However, high-temperature aging can pose a problem of volatilization of azo-based initiators used in the past. In contrast, the azo-based initiator contained in the nonaqueous electrolyte composition according to the present invention is only slightly volatilized before the cyclic carbonate polymer coating is formed, which allows for sufficient initiation reaction and inhibits various side reactions caused by the volatilized initiator.

[0103] The aging may be carried out for 4 hours to 48 hours, preferably 6 hours to 42 hours, and more preferably 8 hours to 36 hours.

[0104] The discharge may be performed until the state of charge (SOC) reaches 0% to 65%, preferably 10% to 60%, and most preferably 15% to 25%. The discharge may be performed at room temperature, for example, 20°C to 30°C.

[0105] The present invention will be described in more detail below with reference to specific examples. However, the following examples are merely illustrative for understanding the present invention and are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the present description and technical concept, and it goes without saying that such changes and modifications fall within the scope of the appended claims.

[0106] [Example] Example 1 (Production of non-aqueous electrolyte) A non-aqueous solvent was prepared by dissolving LiPF6 in an organic solvent (ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 30:70 volume ratio) to a concentration of 1M. 0.5 g of vinylene carbonate and 0.2 g of the azo initiator compound of Formula 1-1 were added to 99.3 g of the non-aqueous solvent to prepare a non-aqueous electrolyte composition.

[0107] [ka]

[0108] (Lithium secondary battery manufacturing) Cathode active material (LiNi 0.8 Co 0.1 Mn 0.1 A cathode slurry was prepared by adding O2, a conductive material (Super-P), and a binder (polyvinylidene fluoride) in a weight ratio of 97.74:0.70:1.56 to a solvent, N-methyl-2-pyrrolidone (NMP). The cathode slurry was applied to one side of a 15 μm-thick cathode current collector (Al thin film), dried, and roll-pressed to prepare a cathode.

[0109] Anode active material (graphite), conductive material (Super-P), and binder (SBR-CMC) were mixed in a weight ratio of 96.15:0.50:3.35 with N-methyl-2-pyrrolidone (NMP) as a solvent to prepare anode slurry. The anode slurry was applied to one side of a 15 μm-thick anode current collector (Cu thin film), dried, and roll-pressed to prepare anode.

[0110] In a dry room, a polyolefin-based porous separator coated with inorganic particles AlO was interposed between the positive electrode and negative electrode prepared above, and the nonaqueous electrolyte composition prepared above was then injected to prepare a lithium secondary battery.

[0111] The fabricated lithium secondary battery was charged at 60° C. to SOC 40%, aged at 60° C. for 24 hours, and then discharged at room temperature to SOC 20% to fabricate a lithium secondary battery.

[0112] Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that a non-aqueous electrolyte solution was prepared by adding 0.5 g of vinylene carbonate and 0.2 g of the azo initiator compound of Formula 1-2 to 99.3 g of the non-aqueous solvent prepared in Example 1.

[0113] [ka]

[0114] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 0.5 g of vinylene carbonate was added to 99.5 g of the non-aqueous solvent prepared in Example 1 to prepare a non-aqueous electrolyte solution.

[0115] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 0.5 g of vinylene carbonate and 0.2 g of azobisisobutyronitrile (ABIN) were added to 99.3 g of the non-aqueous solvent prepared in Example 1 to prepare a non-aqueous electrolyte solution. [ka]

[0116] Comparative Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 5.5 g of vinylene carbonate and 0.2 g of the azo initiator compound of Formula 1-1 were added to 94.3 g of the non-aqueous solvent prepared in Example 1 to prepare a non-aqueous electrolyte solution.

[0117] Comparative Example 4 A lithium secondary battery was manufactured in the same manner as in Example 1, except that a non-aqueous electrolyte solution was prepared by adding 0.005 g of vinylene carbonate and 0.2 g of the azo initiator compound of Formula 1-1 to 99.795 g of the non-aqueous solvent prepared in Example 1.

[0118] [Experimental Example - Evaluation of High-Temperature Storage Characteristics] The high-temperature storage characteristics of each of the lithium secondary batteries produced in Examples 1 and 2 and Comparative Examples 1 to 4 were evaluated.

[0119] Specifically, each of the lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 4 was fully charged to 4.2 V and then stored at 60° C. for 8 weeks.

[0120] Before storage, the capacity and resistance of the secondary battery fully charged at room temperature were measured and set as the capacity and resistance of the initial secondary battery.

[0121] After 8 weeks, the capacity of the stored secondary battery was measured, and the capacity loss and resistance increase during the 8-week storage period were calculated. The percentage of the capacity loss and resistance increase relative to the initial capacity of the secondary battery were calculated to derive the capacity retention rate and resistance increase rate after 8 weeks. The results are shown in Table 1 below.

[0122] [Table 1]

[0123] As shown in Table 1, it was confirmed that the lithium secondary batteries of Examples 1 and 2 had a higher capacity retention rate after 8 weeks and stable performance at high temperatures compared to the secondary batteries of Comparative Examples 1 to 4. In particular, it was confirmed that the lithium secondary batteries of Examples 1 and 2 had a significantly lower resistance increase rate after 8 weeks compared to the secondary batteries of Comparative Examples 1 to 4. This is thought to be because the lithium secondary battery of Comparative Example 1 did not contain an azo-based initiator, and therefore a ring-opening reaction occurred preferentially in vinylene carbonate, preventing the appropriate production of a cyclic carbonate polymer coating formed by a polymerization reaction of carbon-carbon double bonds.

[0124] In addition, it is believed that in the lithium secondary battery of Comparative Example 2, a large amount of azobisisobutyronitrile (ABIN) was volatilized before the cyclic carbonate polymer coating was formed, and the cyclic carbonate polymer coating was not properly formed.

[0125] The lithium secondary battery of Comparative Example 3 had an excessively high cyclic carbonate content, resulting in the formation of an organic SEI layer. A coating with too much organic material is believed to have poor high-temperature stability and prone to breakdown, leading to side reactions at the electrode-electrolyte interface and a decrease in capacity during cycling. Furthermore, CO2 gas generated during the breakdown of the coating is believed to cause swelling of the battery, resulting in an increase in resistance.

[0126] The lithium secondary battery of Comparative Example 4 is considered to be inferior in effectiveness because the content of cyclic carbonate is too small to form a sufficient SEI film.

Claims

1. A non-aqueous electrolyte composition comprising an organic solvent, a lithium salt, a cyclic carbonate-based additive containing a carbon-carbon double bond, and an azo-based initiator, The azo initiator is represented by the following formula 1: The azo initiator is included in an amount of 0.001 wt % to 0.3 wt % based on the total weight of the nonaqueous electrolyte composition; The non-aqueous electrolyte composition contains the cyclic carbonate additive containing a carbon-carbon double bond in an amount of 0.01 wt % to 5 wt % based on the total weight of the non-aqueous electrolyte composition. 【Chemistry 1】 (In the above chemical formula 1, R １ ~R ４ are each independently selected from an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms; R １ ~R ４ (Except when all of are methyl groups.)

2. 2. The nonaqueous electrolyte composition according to claim 1, wherein the organic solvent comprises at least one organic solvent selected from the group consisting of ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate.

3. The lithium salt is LiPF ６ 2. The non-aqueous electrolyte composition of claim 1, comprising one or more selected from the group consisting of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

4. 2. The nonaqueous electrolyte composition according to claim 1, wherein the lithium salt is contained at a concentration of 0.1 M to 4.0 M.

5. 2. The nonaqueous electrolyte composition according to claim 1, wherein the cyclic carbonate additive containing a carbon-carbon double bond is at least one selected from the group consisting of vinylene carbonate and vinylethylene carbonate.

6. 2. The nonaqueous electrolyte composition according to claim 1, wherein the azo-based initiator is a compound represented by the following chemical formula 1-1 or 1-2: 【Chemistry 2】 【Transformation 3】

7. 2. The non-aqueous electrolyte composition according to claim 1, wherein the azo-based initiator is contained in an amount of 0.1 wt % to 0.25 wt % based on the total weight of the non-aqueous electrolyte composition.

8. A lithium secondary battery manufactured by injecting any one of the nonaqueous electrolyte compositions according to claim 1 into a case housing an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

9. preparing a lithium secondary battery filled with a non-aqueous electrolyte composition including an organic solvent, a lithium salt, a cyclic carbonate-based additive containing a carbon-carbon double bond, and an azo-based initiator; activating the lithium secondary battery and forming a cyclic carbonate polymer coating on the surface of the electrode, The azo initiator is represented by the following chemical formula 1: The method for producing a lithium secondary battery, wherein the cyclic carbonate additive containing a carbon-carbon double bond is contained in an amount of 0.01% by weight to 5% by weight based on the total weight of the non-aqueous electrolyte composition. 【Chemistry 4】 (In the above chemical formula 1, R １ ~R ４ are each independently selected from an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms; R １ ~R ４ (Except when all of are methyl groups.)

10. 10. The method for manufacturing a lithium secondary battery according to claim 9, wherein the organic solvent comprises at least one organic solvent selected from the group consisting of ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate.

11. 10. The method for manufacturing a lithium secondary battery according to claim 9, wherein the cyclic carbonate additive containing a carbon-carbon double bond is at least one selected from the group consisting of vinylene carbonate and vinylethylene carbonate.

12. The method for manufacturing a lithium secondary battery according to claim 9, wherein the azo-based initiator is a compound represented by the following Chemical Formula 1-1 or 1-2: 【Transformation 5】 【Transformation 6】

13. 10. The method for manufacturing a lithium secondary battery according to claim 9, wherein the azo-based initiator is contained in an amount of 0.001 wt % to 0.3 wt % based on the total weight of the non-aqueous electrolyte composition.

14. The method for manufacturing a lithium secondary battery according to claim 9 , wherein the activation step includes an initial charging process, an aging process, and a discharging process.

15. The method for manufacturing a lithium secondary battery according to claim 14, wherein the aging process is performed at 30°C to 80°C.

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